Fix atexit on Python 3 (#1852)

* Fix atexit on Python 3

`sys.exitfunc` was removed in Python 3, meaning that any handlers
registered with the `pwnlib/atexit.py` module wouldn't run. This PR
reigsters the handlers with the stdlib atexit.

* Update changelog

Author: clubby789

CHANGELOG.md

@@ -151,6 +151,7 @@ The table below shows which release corresponds to each branch, and what date th
 
 ## 4.3.1
 
+- [#1852][1852] Fix `atexit` on Python 3
 - [#1732][1732] Fix shellcraft SSTI vulnerability (first major pwntools vuln!)
 
 [1732]: https://github.com/Gallopsled/pwntools/pull/1732

pwnlib/atexit.py

@@ -13,6 +13,7 @@
 import sys
 import threading
 import traceback
+import atexit as std_atexit
 
 from pwnlib.context import context
 
@@ -95,4 +96,8 @@ def _run_handlers():
 if hasattr(sys, "exitfunc"):
     register(sys.exitfunc)
 
-sys.exitfunc = _run_handlers
+if sys.version_info[0] < 3:
+    sys.exitfunc = _run_handlers
+else:
+    std_atexit.register(_run_handlers)
+